board/teensy40: Add I2C support

[ardnew]

This PR is baselined on top of the Teensy 4.0 UART support PR (#1454)

This PR adds support for 3 (three!) I2C interfaces on the Teensy 4.0.

The public I2C API is the same as most other TinyGo devices with no substantial deviations I'm aware of.

Testing this implementation has been done entirely through 2 new drivers I'm actively developing and are not yet available.

The following table (from src/machine/board_teensy40.go) succinctly describes all of the important properties of each of the 3 supported I2C interfaces available in TinyGo:

// #====================================================#
// |                         I2C                        |
// #===========#==========#=============#===============#
// | Interface | Hardware | Clock(Freq) | SDA/SCL : Alt |
// #===========#==========#=============#=========-=====#
// |   I2C1    |  LPI2C1  | OSC(24 MHz) | D18/D19 : 3/3 |
// |   I2C2    |  LPI2C3  | OSC(24 MHz) | D17/D16 : 1/1 |
// |   I2C3    |  LPI2C4  | OSC(24 MHz) | D25/D24 : 0/0 |
// #===========#==========#=============#=========-=====#

[deadprogram]

Please rebase this branch against dev to resolve merge conflicts. Thanks.

[ardnew]

rebased against dev

[ardnew]

rebased against dev

[deadprogram]

Sorry I have not looked at this PR sooner. I will get out my Teensy 4.0 and try it out in a bit.

[deadprogram]

Was trying this PR but #1454 (comment)

General comment: should define a I2C0 to point to I2C1 in this PR?

[ardnew]

rebased against dev (3eb9dca)

[sago35]

@ardnew
Both i2c0 and i2c2 worked, but i2c4 did not.
I think it needs a few fixes, but for the most part it is very good.

[deadprogram]

@ardnew if you get a chance to respond to @sago35 feedback on this board it would be great. Thank you!

[ardnew]

Addressed remaining comments and rebased against dev (b5c5d95)

[deadprogram]

Since #2924 passed CI I will squash/merge this PR now. Thank you very much @ardnew